"1940 The Smith Act is passed. The Act makes it a crime to advocate the violent overthrow of the government, to distribute any material that teaches or advocates such, or to belong to a group with such an aim."

http://www.infoplease.com/timelines/freespeech.html

This was the justification for persecuting Communists. The US persecuted not only those who advocated the violent overthrow of the government but also those who sympathized with them.

There aren't many crimes more heinous.

This is in addition to "aid and comfort to the enemy" and that sort of thing. I presume there will be a line of Conservatives repudiating Rush, as they did when they wished the government to fail.